To date, as a manufacturing method of a pleated air filter pack into which a filter medium is folded in zigzag shape, there has been proposed a method, disclosed in Patent Document 1, whereby a resin which is an interval maintaining material is applied in constant thickness on either surface of at least each side portion of the filter medium, forming resin layers, after which the filter medium is folded in zigzag shape at regular intervals, and the resin layers are solidified, thus making the intervals between the folds of the filter medium constant.
With the heretofore described method, as the obtained zigzag-shaped filter medium for an air filter is such that the filter medium allowing air to pass through is folded at regular intervals, the flow direction of the air entering from one side of the filter medium, when discharged from the other side of the filter medium, is perpendicular to the filter medium, meaning that it is not possible to obtain a predetermined airflow pressure unless the pressure of the air caused to pass through is increased.
In order to solve this problem, a method of providing pleats with a tapered form by applying a hot melt adhesive in layers and forming a gradient from a ridge to a valley of the crease is proposed in Patent Document 2.
Also, a method of maintaining the intervals between pleats using projections formed by an embossing is proposed in Patent Document 3.